Method and apparatus for uniform heating in a microwave oven

ABSTRACT

A microwave oven and a method for heating a load which is placed therein. According to the invention a predetermined mode in the cavity of a microwave oven is fed by means of an associated feeding port which is arranged to feed essentially the intended mode only, the feeding of a mode other than the intended, predetermined mode being essentially prevented.

TECHNICAL FIELD

The present invention generally relates to the field of microwave ovensand, more particularly, to the feeding of microwaves to a cavity in amicrowave oven for heating food which is placed in said cavity.

TECHNICAL BACKGROUND

When heating a load in the form of food by means of a microwave oven,there are a number of aspects which have to be considered. Most of theseaspects are well-known to those skilled in the art and include, forinstance, the desire to obtain uniform heating of the food at the sametime as a maximum amount of available microwave power is absorbed in thefood with a view to achieving a satisfactory degree of efficiency.

In order to achieve uniform heating of the load, microwave stirrersand/or a rotating plate; on which the load is to be placed, have earlierbeen used.

In order to provide efficient coupling of microwaves to the cavity in amicrowave oven, it has previously been suggested that a microwave sourcehaving a controllable frequency might be used. U.S. patent specificationU.S. Pat. No. 4,196,332, for example, discloses such a microwave oventhat works according to a predetermined pattern. First a frequency scanwithin a predetermined range is carried out, during which reflectionsfrom the cavity are detected, and the frequencies which give the lowestreflection are stored in a memory. Then the cavity of the microwave ovenis fed with microwaves of a predetermined frequency which gives a lowdegree of reflection. The same specification also suggests jumpingbetween a plurality of more or less optimal frequencies.

SUMMARY OF THE INVENTION

A general object of the present invention is to provide a microwave ovenin which, on the one hand, the heating of a load in the oven is morehomogeneous, and, on the other hand, the heating of the load in relationto available microwave power is greater than that allowed by prior-artmicrowave ovens.

This object is achieved by means of a microwave oven and methods inconnection with the same of the type stated in the appended claims.

Microwave ovens according to prior-art technique use relativelybroadband microwave sources which are adapted to feed energy to amaximum part of the cavity of the microwave oven and excite a largenumber of modes and, thus, provide heating of a load that is placed inthe cavity. However, interference between the modes in the cavityresults in places with undesirably-low energy density and places withundesirably high energy density. Such places ate sometimes called “coldspots” and “hot spots”, respectively, since the heating of the load inthese places becomes too low and too high, respectively. To a certainextent, the present invention is based on an understanding of hownarrow-band microwave sources operate in a microwave oven. In fact, itis possible to excite, by means of a narrow-band microwave source, oneand only one mode in the cavity and, thus, obtain excellent control ofthe energy distribution in the cavity on condition that the microwavesare fed to the cavity by means of carefully positioned feeding ports.Further mode selectivity in connection with feeding is obtained by theemission frequency of the microwave source being selected so that itsuits the intended mode in the cavity. In this connection, a microwavesource is thus considered to be of a narrow-band type if it emitsmicrowaves within a frequency range which is so small that excitation ofessentially one predetermined mode in the cavity is allowed. Thelocation of feeding ports in accordance with the present invention willbe discussed at length in the following detailed description of a numberof preferred embodiments of the invention.

The following description of the invention relates to, above all, amicrowave oven which operates with microwaves in the frequency range2.4-2.5 GHz, which is a common frequency range as regards microwaveovens for household use. Also other frequencies can be used formicrowave heating (e.g. frequencies around 915 MHz) and the presentinvention is, of course, also applicable within these other frequencyranges.

Preferably, the narrow-band microwave generator used in the presentinvention is a solid-state based microwave generator that comprises, forexample, silicon carbide components (SiC). The advantages of asolid-state based microwave generator comprise the possibility ofcontrolling the frequency of the generated microwaves, controlling theoutput power from the generator and an inherent narrow-band feature. Thefrequencies of the microwaves that are emitted from a solid-state basedgenerator usually constitute only a very narrow range of theabove-mentioned available range of 2.4 to 2.5 GHz.

The invention is thus based on the understanding of how the modesallowed by the cavity of a microwave oven can be used in order toachieve efficient and uniform heating of a load in the cavity.

In line with the fundamental inventive concept, there is also theunderstanding of the advantages of feeding microwaves to the cavity bymeans of one or more feeding ports in the enclosing surface of thecavity, where each feeding port is arranged to feed energy toessentially one predetermined mode in the cavity. In some cases, afeeding port that is not used at the moment is preferablyshort-circuited, thus preventing the microwave energy from being let outof the cavity via said feeding port.

Moreover, the invention is based on deeper understanding of how a modein the cavity of the microwave oven is changed or becomes distorted whena load is placed in the cavity.

The mode pattern in the cavity of a microwave oven does not look thesame when the cavity is empty as in the case where it contains a load.When a load is placed in the cavity, the mode pattern will, however,change in a way that in some sense is predictable. Usually, the modepattern, although a load is placed in the cavity, is essentiallyunchanged in at least one dimension (height, width or depth).

It should be noted that when the expression “mode” is used in thefollowing, it is related to the field pattern in the cavity as itappears when the cavity is empty, that is in a cavity without a load. Inthe light of these facts, it is easy to understand what is meant by, forinstance, the terms “distorted”, or “altered”, mode.

According to one aspect of the invention, a microwave oven is provided,which comprises a cavity that is fitted with at least one feeding port,through which microwaves are to be fed, the feeding port being arrangedto feed energy to one single predetermined mode in the cavity. In otherwords, feeding of energy through said port to a mode other than thepredetermined mode is essentially prevented.

According to another aspect of the invention, a microwave oven isprovided, which comprises a cavity that is fitted with at least twofeeding ports, the feeding ports being arranged in different places, sothat a predetermined mode in the cavity is feedable in at least twodifferent points, while feeding of energy through said ports to anothermode than the predetermined mode is essentially prevented. If a load isplaced in the cavity and the mode pattern is thus distorted in such amanner that one of the feeding ports can no longer couple energy to thepredetermined mode in an efficient way, the second feeding port willunder some circumstances still work efficiently enough. Consequently,efficient feeding of microwaves to the cavity can be maintained alsowhen the mode pattern is changed due to the presence of a load in thecavity by controlling the feeding of microwaves to a feeding port whichgives a large coupling factor when feeding microwaves to the distortedmode.

According to another aspect of the invention, a microwave oven isprovided, which comprises a cavity that is fitted with feeding ports atsuch locations that at least two modes are feedable while each feedingport only feeds a predetermined mode in the cavity. By controlling saidfeeding ports, the distribution of energy between said modes may thus bemade to promote uniform heating of a load in the cavity. Preferably,said modes in the cavity have essentially non-overlapping heatingpatterns.

According to yet another aspect of the invention, a method is providedfor heating, by means of energy in the form of microwaves, of a load inthe cavity of a microwave oven, a first feeding port feeding energy to afirst predetermined mode in the cavity, and feeding of energy from saidfirst feeding port to another mode than the first predetermined mode isessentially prevented.

According to another aspect of the invention, a method is provided forheating, by means of energy in the form of microwaves, of a load in thecavity of a microwave oven, a first feeding port feeding energy toessentially only a first predetermined mode in the cavity, and a secondfeeding port feeds energy to essentially only a second predeterminedmode in the cavity, the modes preferably having essentiallynon-overlapping heating patterns.

In an example of a microwave oven which has a design and a function inaccordance with the present invention, the cavity is formed to support apredetermined set of modes. The cavity is furthermore fitted with aplurality of feeding ports, each feeding port essentially feeding energyto one associated mode in the cavity. When the microwave oven is inoperation, said feeding ports are controlled so that uniform heating ofa load placed in the cavity is promoted. The control may be performed onthe basis of a plurality of various conditions, such as the temperaturedistribution in the load or the reflected power from the feeding ports.This control may, for instance, be performed by monitoring, by means ofIR sensors, the actual temperature distribution across the load or bymeasuring the microwave power which is reflected from one or more of thefeeding ports, the feeding being directed to such feeding ports as givean even temperature distribution and a low reflected power. The variousmodes in the cavity can also, owing to the fact that each feeding portfeeds essentially one associated mode, be fed sequentially, whereby thepossibility of controlling the energy distribution in the cavity isfurther increased.

A great advantage of the present invention is thus that a substantiallymore uniform heating of a load placed in the cavity of a microwave ovenis provided. Unlike prior-art technique, it is possible by means of thepresent invention to eliminate the formation of hot and cold spots inthe cavity to a very large extent. This uniform heating is allowed bythe fact that the mode pattern in the cavity is controlled by means ofmicrowave feeding via carefully positioned feeding ports. At least onefeeding port then has the property that only one predetermined mode inthe cavity can be fed from said feeding port. Consequently, anuncontrolled energy distribution in the cavity is efficiently avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following a number of preferred embodiments of the invention willbe described in more detail. In the detailed description references aremade to the accompanying drawings, in which

FIG. 1 a is a general view of a microwave oven which has features andfunctions in accordance with the present invention,

FIG. 1 b shows a block diagram of the function of the microwave ovenshown in FIG. 1 a,

FIG. 2 schematically shows a first cavity having a first feeding port,

FIG. 3 schematically shows the mode pattern that is formed in connectionwith excitation from the feeding port shown in FIG. 2,

FIG. 4 schematically shows a second cavity having a second feeding port,

FIG. 5 schematically shows the mode pattern which is formed inconnection with excitation from the feeding port shown in FIG. 4,

FIG. 6 schematically shows a cavity which is provided with four feedingports for excitation of four different modes in the cavity,

FIG. 7 is a schematic top plan view of the mode TM₄₁₂ in a cavitywithout a load,

FIG. 8 is a schematic top plan view of the mode which is shown in FIG. 7and which is now distorted due to the presence of a load in the cavity,

FIG. 9 is a schematic cross-sectional view of the mode shown in FIG. 7,

FIG. 10 is a schematic cross-sectional view of the distorted mode shownin FIG. 8,

FIG. 11 schematically shows a second example of a cavity which isprovided with four feeding ports for excitation of four different modesin the cavity,

FIG. 12 schematically shows a row of IR sensors for detecting thetemperature distribution in a load which is placed in the cavity of amicrowave oven,

FIG. 13 schematically shows a first heating pattern in the load,

FIG. 14 schematically shows a second heating pattern in the load,

FIG. 15 schematically shows how microwave feeding can be controlledbetween three different feeding ports,

FIG. 16 schematically shows a first preferred example of a feeding port,

FIG. 17 schematically shows a second preferred example of a feedingport,

FIG. 18 schematically shows an arrangement for allowing measurement ofthe microwave power which is reflected from the feeding port,

FIG. 19 schematically shows the power distribution (heating patterns) ina load as regards a first mode, and

FIG. 20 schematically shows power distribution (heating patterns) in aload as regards a second mode.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 a is a general view of a microwave oven 1 which has features andfunctions according to the present invention. The microwave ovencomprises a microwave generator 10 which is operatively connected to aplurality of feeding ports 12 (in the shown example, four), throughwhich microwaves are to be fed to the cavity 15 of the microwave oven.The generator 10 is connected to the feeding ports 12 by means of atransmission line 13 which connects to the feeding ports via a switch 16associated with a respective feeding port. These switches can, whilebeing controlled by a control unit 18, stop the feeding from arespective feeding port, whereby feeding from the intended feeding portsonly is allowed. The switches 16 can also comprise means for measuringthe microwave power that is reflected from each feeding port. The resultof such a measurement is transmitted to the control unit 18 which usesthe measurements to control the microwave feeding to suitable feedingports. Moreover, a row of IR sensors 20 is preferably arranged inconnection with the cavity, with the aim of measuring the temperaturedistribution in a load (not shown) that is placed in the cavity. Alsothe result of such a measurement is transmitted to the control unit 18and is used for controlling the microwave feeding to suitable feedingports. The four feeding ports shown in the figure are arranged to feedone predetermined mode each in the cavity 15. With the purpose ofensuring feeding of only one predetermined mode from each feeding port,the ports 12 are placed in such a manner that the coupling factor to thecavity is large only for the predetermined mode. This can be provided byplacing the ports at locations where only one mode exhibits a largeamplitude, by orientating the ports so that excitation of the correctpolarisation is obtained, and by tuning the emission frequency of themicrowave generator so as to fit the intended cavity mode.

The general function of the microwave oven shown in FIG. 1 a isillustrated in FIG. 1 b in the form of a block diagram. The generator 10feeds microwaves to the four feeding ports 12 via an associated switch16. The switches are controlled by the control unit 18. In addition toparameters indicated by a user, the control may also take place on thebasis of measured conditions in the microwave oven. In FIG. 1 b twodifferent examples of these are shown. Firstly, the controlling may takeplace on the basis of measured temperature distribution in the loadplaced in the cavity, the measurement being performed by means of a rowof IR sensors 20. Secondly, the control may take place on the basis of ameasurement, by means of detectors 22 of a type well known to thoseskilled in the art, of microwave power which is reflected from at leastone feeding port. The results of the measurements are transmitted to thecontrol unit 18 which interprets and uses these results for controllingthe microwave generator 10 as well as the switches 16.

As pointed out above, the present invention is based on an understandingof how separate modes in the cavity of a microwave oven can be excitedselectively by carefully arranging feeding ports. Preferably, but notnecessarily, the feeding of microwaves from the microwave generator ofthe microwave oven to the cavity takes place by means of one or moretransmission lines, for instance, striplines or microstrips, in whichcase the feeding ports comprise an H-loop or a slot in the ground planeof the transmission line. The preferred embodiment of the feeding portswill be described in more detail below. Feeding of the modes in thecavity thus takes place via the magnetic field of these modes, and,therefore, the feeding ports are preferably arranged at such locationswhere corresponding modes exhibit an amplitude maximum for the magneticfield (H-field).

In FIGS. 2 and 4 two preferred arrangements of feeding ports are shownschematically in accordance with the present invention, and in FIGS. 3and 5, the magnitude of the magnetic field is shown for correspondingcavity modes in the horizontal plane. The cavity in the example shown inFIGS. 2-5 has the dimensions b=327 mm, d=327 mm, h=189 mm and isresonant for, inter alia, the modes TM₅₁₁ and TM₄₁₂. The feeding port200 shown in FIG. 2 is placed on the surface x=0, at y=d/2, z=h/2 andfeeds microwaves essentially only to the cavity mode TM₄₁₂, while thefeeding port 400 shown in FIG. 4 is placed on the surface z=h (the “top”of the cavity), at x=3*b/5, y=d/2 and feeds microwaves substantiallyonly to the cavity mode TM₅₁₁. FIGS. 3 and 5 in reality show the resultof actual simulations of the resulting energy distribution in the cavitywhen using the arrangements of the feeding ports shown in FIGS. 2 and 4.It is evident from the figures that excitation of other modes than theintended mode is negligible. In order to make this type of selectivefeeding of microwaves to selected modes in the cavity advantageous, itis thus necessary to carefully design the cavity with well chosendimensions, so that the modes for which the cavity is resonant becomeunambiguous and well known.

In line with the above reasoning, it is possible to go further andarrange feeding ports which correspond to various desired modes in thecavity. Examples of other arrangements of feeding ports are shown inFIG. 6. The dimensions of the cavity are in this example 327×350×189 mm³(i.e. b=327 mm, d=350 mm and h=189 mm), which gives resonance for themodes TM₅₁₁, TM₄₁₂, TM₃₃₂ and TM₂₅₁. Since each one of the feeding portsis arranged at a location where only one mode in the cavity exhibits alarge, or substantially maximum amplitude of the magnetic field for thecomponent which is excited, the intended situation is brought about, inwhich each feeding port feeds microwaves to essentially onepredetermined mode only, while feeding of microwaves to a mode otherthan the predetermined mode is substantially prevented. Feeding portsare placed as follows by means of a system of co-ordinates as shown inthe figure. The feeding port 12.1 of the mode TM₅₁₁ is placed on theenclosing surface at z=h (the “top” of the cavity) at x=4*b/5, y=d/2.The feeding port 12.2 of the mode TM₄₁₂ is placed on the enclosingsurface at x=b and is centred on this surface at y=d/2, z=h/2. Thefeeding port 12.3 of the mode TM₃₃₂ is placed on the enclosing surfaceat z=h at x=b/3, y=d/6. The feeding port 12.4 of the mode TM₂₅₁ isplaced on the enclosing surface at z=h at x=b/4, y=4*d/5. Thesearrangements of the feeding ports 12.1, 12.2, 12.3 and 12.4 are chosenin such a manner that mainly one predetermined mode in the cavity can beexcited by means of a respective feeding port. Naturally, theabove-mentioned modes can also be excited from other locations, but thenthere is also a risk of undesired modes in the cavity being excited.

FIG. 7 is a top plan view of the cavity of a microwave oven, the cavityhaving the dimensions 357×327×327×189 mm³. In the figure the amplitudeof the magnetic field is shown as regards a mode TM₄₁₂ (magnitude ofH_(y)), which is generated in the cavity, in a plane 20 mm above thebottom of the cavity. Areas having a large amplitude 71 aresubstantially symmetrically distributed in the x-direction, separated byareas having a small amplitude 72. Between these areas 71, 72 theamplitude varies continuously. Efficient feeding of microwaves to thecavity may, as is evident from the figure, take place both from the leftenclosing surface 75 in the figure and from the right enclosing surface77 in the figure thanks to the fact that the amplitude of the magneticfield at these locations exhibits a maximum. Since the shown mode isTM₄₁₂, efficient feeding may also occur from locations close to thecentre of the wall of the cavity and close to the top of the cavity. Itwill thus be appreciated that the mode TM₄₁₂ exhibits three maxima ofthe magnetic field in the z-direction. However, with a view to avoidingexcitation of undesired modes in the cavity, feeding preferably takesplace from a feeding port which is placed close to the centre of thewall of the cavity.

FIG. 8 once again shows the cavity shown in FIG. 7 but now with a loadpresent. In the figure the load is illustrated in the form of arectangle 80. The load causes a distortion of the mode pattern on acomparison with the appearance in connection with an empty cavity (cf.FIG. 7). The figure shows the amplitude (magnitude of H_(y)) of themagnetic field in a horizontal plane on a level with the upper side ofthe load, in this case 20 mm above the bottom plane of the cavity. Thefigure shows that the mode pattern is significantly distorted, and,therefore, areas having a large amplitude 81 and areas having a smallamplitude 82 seem different from the case where the cavity is empty. Iffeeding of microwaves in this case would take place by means of, forexample, an H-loop which is arranged to couple to the y-component(H_(y)) of the magnetic field, feeding to the distorted mode patternstill occurs efficiently enough from the right 87 or left 85 enclosingsurface in the figure somewhat above the bottom of the cavity (along they-axis close to the centre of the enclosing surface). Even if the figureshows a mode pattern in a plane 20 mm above the bottom of the cavity, itwill be appreciated that the mode pattern also becomes distorted in aplane at half the cavity height. With the aim of avoiding excitation ofundesired modes in the cavity, the feeding port of the mode TM₄₁₂ on theenclosing surface in question is, however, as pointed out earlier,preferably placed close to the centre of this surface. In other cases,the mode pattern will be distorted in another way. Various types ofloads give, of course, different distortions of the mode pattern, ofwhich the shown distortion is one example, but independent of how themode pattern is changed when a load is placed in the cavity, the use ofseveral feeding ports for one and the same predetermined mode willresult in increased possibility of feeding microwaves to the mode. Theadvantageous function of a microwave oven with feeding according to thepresent invention is evident, so that an efficient enough feeding ofmicrowaves to the cavity is allowed also in a distorted mode pattern dueto the fact that one and the same predetermined mode in the cavity isfed from two or more feeding ports which are placed at differentlocations in the enclosing surface of the cavity. Preferably, thedifferent feeding ports of one and the same mode are placed on sides ofthe cavity wall which are orthogonal in relation to one another.

With the purpose of avoiding that the microwave power is coupled fromthe cavity through a feeding port which at a certain point of time isnot in operation, it is preferred that the feeding ports, when not inoperation, are short-circuited by means of, for instance, a circulatorand a switch, whereby coupling from the cavity through theshort-circuited feeding port is prevented. If two or more feeding portsare used for feeding one and the same predetermined mode, it may, ofcourse, happen that two or more of these feeding ports after all have ahigh coupling factor to the cavity mode, even though a load is placed inthe cavity. If the feeding port then does not feed microwaves to thecavity, there is a risk of microwaves instead slipping out of the cavitythrough said feeding port, unless the feeding port is short-circuited inaccordance with the above.

FIG. 9 shows the same cavity as FIG. 7 but in the x-z-plane. The modepattern is shown in one plane at half the cavity depth, that is in theplane where the feeding ports are to be placed according to thediscussion in connection with FIGS. 7 and 8. FIG. 10 shows, in acorresponding way, the distorted mode pattern.

FIGS. 9 and 10 show that locations can be found, from which feedingtakes place efficiently by the non-distorted mode as well as thedistorted mode. Areas having a large amplitude 91, 101 are separated byareas having a small amplitude 92, 102 and it is evident from thefigures how the mode pattern changes at the presence of a load in thecavity. FIGS. 9 and 10 show that an efficient feeding may occur from,for example, the right side 97, 107 of the figure, since the amplitudeof the magnetic field is large there. The figures also show a feedingport 95, 105, which is placed on the enclosing surface 97, 107 to theright in the figure. In order to provide truly reliable feeding of apredetermined mode in the cavity both when the mode is distorted andwhen the mode is non-distorted, advantageously, it is possible to placefeeding ports on all enclosing surfaces that are orthogonal in relationto one another and then control the feeding to such ports that have ahigh coupling factor to the cavity mode. Once again the inventivefunction is evident: at least one feeding port usually functionssatisfactorily also when the mode pattern is distorted due to thepresence of a load in the cavity.

FIG. 11 shows yet another cavity of the microwave oven which is formedin accordance with the present invention. In this case, the cavity has asquare cross-section in the horizontal plane. The dimensions of thecavity are b=327 mm, d=327 mm and h=189 mm, which make the cavityresonant to the modes TM₁₄₂, TM₄₁₂, TM₁₅₁ and TM₅₁₁. The feeding portsof the four modes mentioned above are placed as follows in a system ofco-ordinates as that shown in the figure: the feeding port 12.5 of themode TM₄₁₂ is placed on the enclosing surface x=b and is centred on thissurface at y=d/2, z=h/2. The feeding port 12.6 of the mode TM₁₄₂ isplaced on the enclosing surface y=d and is centred on this surface atx=b/2, z=h/2. The feeding port 12.7 of the mode TM₁₅₁ is placed on theenclosing surface z=h (the “top” of the cavity) at x=b/2, y=2*d/5. Thefeeding port 12.8 of the mode TM₅₁₁ is also placed on the enclosingsurface z=h, but at x=3*b/5, y=d/2.

Both in FIGS. 6 and 11 the feeding ports are shown in the form ofrectangles on the enclosing surface of the cavity. This indicates thatthere is yet another degree of freedom when placing the feeding ports,in addition to the locations at which the ports are placed, namely theorientation of the ports. The feeding ports couple microwaves to themodes of the cavity in a polarised manner, that is, either to thex-component of the magnetic field or to its y-component. It is easilyappreciated by those skilled in the art that an H-loop or a slot in theground plane of a transmission line connects to a predeterminedcomponent of the magnetic field. Thus, yet another possibility toprevent that a certain feeding port feeds microwaves to a mode in thecavity other than the intended mode.

FIGS. 12-14 illustrate how sequential use of two cavity modes are usedin order to obtain more uniform heating of a load placed in the cavity.

FIG. 12 schematically shows a row of IR sensors which may be used formeasuring the temperature distribution in the load. A first group ofsensors, A_(i), measures the temperature in a first subset of the loadand a second group of sensors, B_(i), measures the temperature in asecond subset of the load. Even if the measuring of the temperaturedistribution in this case only takes place in one dimension (along thex-axis), it is understood that a similar measuring just as well may takeplace in two dimensions (along the x-axis and y-axis).

The first subset of the load is mainly heated by means of a first cavitymode A, which is illustrated in FIG. 13. In a similar way FIG. 14illustrates how a second subset of the load is heated mainly by a secondcavity mode B. The parts of the load which are heated in the respectivecases are shown in the form of dashed areas in the shown rectangularload.

Below, an itemised example is described of how the heating of the loadcan be controlled by means of the temperature distribution for thesituation shown in FIGS. 12-14.

1. Start the heating by feeding the first mode A (i.e. heating the firstsubset of the load, the temperature of which is measured by the IRsensors A_(i)) during a time Time_(A)=10 s.

2. Measure the temperature distribution by means of the row of IRsensors.

3. Calculate a first average temperature t_(A) by averaging themeasurement results from the sensors A_(i).

4. Calculate a second average temperature t_(B) by averaging themeasurement results from the sensors B_(i).

5. If current feeding takes place to the first mode A: if the cycle timeof A has expired (current time>Time_(A)) or if t_(A)>1.5*t_(B), changeto feeding of the mode B during a time Time_(B)=10 s.

6. If current feeding takes place to the second mode B: if the cycletime of B has expired (current time>Time_(B)) or if t_(B)>1.5*t_(A),change to feeding of the mode A during a time Time_(A)=10 s.

7. Repeat the steps 2-7 every second until the heating is ready.

The heating may, for example, be interrupted after a predetermined, settime or when one or both of the average temperatures t_(A) and t_(B)reach a predetermined value.

Note that in connection with heating according to the above schedulesequential feeding of the cavity modes is used. At an arbitrary point oftime only one of the modes is thus energised, and, therefore, crosstalkbetween the modes are efficiently avoided. In the shown example a simpleand non-distorted mode pattern is used by way of example, the modepattern giving heating patterns which are easy to illustrate in thefigures. In connection with the FIGS. 19-21 a similar situation will bedescribed, but with a considerably distorted mode pattern.

FIG. 15 schematically shows a type of switch 150 which advantageously isused in a microwave oven according to the present invention. The switch150 comprises a circulator 152 with four terminals: an input 155 forfeeding microwaves to the circulator 152 and three outputs 156, 157 and158 for feeding microwaves to three different feeding ports (not shown).Moreover, the switch 150 comprises a feedback loop on one or more of theoutputs 156, 157 and 158. When the feedback loop is closed, a reflectionfrom the output arises which prevents microwaves from slipping outthrough the output. The feedback loop is closed, for instance, by meansof solid-state switches 153, 154, the function of which is controlled bymeans of control signals which each are fed to an associated controlinput 151. By using a switch of the type shown in FIG. 15, it ispossible to easily connect a microwave generator to at least threedifferent feeding ports. In the cases of a microwave generator withvariable frequency, it is also possible to tune the frequency as theswitch changes to a new feeding port, so that an optimum feedingfrequency is obtained for the respective feeding ports.

FIG. 16 shows a first embodiment of a feeding port according to thepresent invention. In the shown example microwaves are led to thefeeding port by means of a transmission line 161 in the form of amicrostrip, which is outside the enclosing surface of the cavity. Theconducting plane of the microstrip is at one location 162short-circuited with the ground plane, which results in the microwaveswhich propagate in the line 161 being reflected at said short circuit162; a standing wave is formed in the transmission line 161. At adistance from said short circuit corresponding to half a wavelength inthe microstrip, a slot 163 is formed in the ground plane and in theenclosing surface of the cavity. At said distance from the shortcircuit, the standing wave exhibits a maximum current in thetransmission line 161 and, thus, the magnetic field also exhibits amaximum at this location. As usual the magnetic field extends, ofcourse, circularly about the line and will thus be let into the cavitythrough said slot 163. In this case, excitation of one mode will takeplace, the mode having the magnetic field directed parallel to theextension of the slot 163.

In FIG. 17 a second embodiment is shown of a feeding port according tothe present invention. Unlike the preceding case with a slot in theground plane/the cavity wall, the conducting plane 171 is nowshort-circuited with the ground plane 172 in the form of a loop 173which reaches through an opening 174 in the cavity wall. Current willthus bypass the loop 173 and, consequently, induce a magnetic fieldtransversely to the plane of the loop. Said loop is therefore namedH-loop since coupling occurs to the H-field in the cavity. In this case,excitation of a mode will occur, having the magnetic field directedperpendicular to the plane of the H-loop 173.

Microwave feeding by means of one of the described feeding ports thusgives coupling to only one of the components of the magnetic field and,in accordance with that discussed earlier, consequently yet anotherpossibility of exciting primarily one single predetermined mode in thecavity.

If the coupling to the cavity is not perfect, some microwave power willbe reflected back through the feeding port, back into the transmissionline. An advantageous, and thus preferred, way to control whether thereis a satisfactory coupling to the cavity, is by measuring the power thatis reflected from the feeding port. FIG. 18 shows a preferred example ofhow such measuring may be provided in the case with one feeding portwhich comprises a slot 183 in the ground plane. A directional coupler181 is arranged adjacent to the transmission line 182 above, that isup-stream of, the slot 183. The directional coupler 181 is in the formof a line that runs parallel to the transmission line 182 across adistance which corresponds to a quarter of the wavelength of themicrowaves in the line 182. A potential microwave power that propagatesup-stream of the slot 183 will thus be disconnected via the directionalcoupler 182 and may subsequently be measured in an already known manner.The result of such a measurement may be used to control the feeding tothe cavity. When the reflected power, for instance, exceeds a limitingvalue, the feeding of microwaves can be directed to an alternativefeeding port, where the reflection is lower. By such control, it ispossible to avoid that too much power is wasted, and, thus, also improvethe efficiency of the entire microwave oven.

If microwave feeding is allowed to continue to a feeding port, fromwhich a large amount of reflection takes place, the reflected power hasto be taken care of in one way or another. Either the microwaves areallowed to propagate back to the microwave generator, or the microwavesare allowed to be transformed to waste heat in some type of microwavesink. It is obvious that both these alternatives are undesirable. Greatadvantages are thus obtained by being able to control the feeding toports having a low reflection.

Preferably, the feeding of microwaves is controlled in such a mannerthat the total reflected power from the cavity is reduced to a minimum,the largest possible ratio of available microwave power thus being usedfor heating of the load placed in the cavity.

Preferably, one or more microwave generators (microwave sources) havinga variable emission frequency is used in embodiments of the presentinvention.

In the cases when only one microwave source is used, the microwavesource may drive, at one and the same point of time, a plurality offeeding ports or one feeding port only. One advantage which is allowedwith a variable emission frequency is that the coupling to apredetermined mode in the cavity can be exactly adjusted by tuning theemission frequency of the microwave source. For instance, in the casewith a distorted mode caused by the presence of a load in the cavity, itis possible to achieve an essentially perfect connection to the cavityby tuning the frequency to the distorted mode.

In other cases a plurality of feeding ports are driven sequentially byone and the same microwave source. It is then possible to adapt theemission frequency to the feeding port (i.e. the cavity mode) which, atone moment, is fed with microwaves and, thus, for each feeding portprovides the highest possible coupling to the intended cavity mode. Whenthe feeding is switched to a new feeding port, also a potential tuningof the emission frequency is performed.

In a microwave oven with a plurality of microwave sources having avariable emission frequency, naturally, even greater possibility ofcontrolling the mode pattern in the cavity is obtained. For instance,simultaneous feeding is allowed of the cavity with two differentfrequencies.

Yet another preferred embodiment is characterised in that apredetermined microwave source is adapted to feed a predetermined modein the cavity by means of at least two feeding ports which are placed atdifferent locations, so that said predetermined mode in the cavity isfeedable in at least two separate points. The possibility of efficientfeeding of the cavity in a considerably distorted mode (caused by thepresence of a load in the cavity) is thus further increased due to thefact that it is also possible to choose, in addition to being able tochoose a suitable feeding port, an emission frequency that is convenientfor the distorted mode.

FIGS. 19 and 20 show the temperature distribution in a rectangular loadseen from above when heating by means of a first and a second cavitymode, respectively. The temperature is shown by contour plots 190, 200(isotherms).

FIG. 19 shows the temperature distribution in the load when only thefirst mode is used for the heating. The mode in the shown example isTM₄₁₂ and it is clear that hot spots appear in the centre 191 of theload in the middle of the edge 192 to the left in the figure andadjacent to the upper 193 and lower 194 corner of the edge to the rightin the figure. At the same time cold spots appear to the right 195 andto the left 196 of the centre 191 of the load.

FIG. 20 shows the temperature distribution in the load when only thesecond mode is used for the heating.

The mode in the shown example is TM₅₁₁, and it is evident that hot spotsappear adjacent to the upper 201 and the lower 202 corner of the edge tothe left in the figure and in the centre of the edge 203 to the right inthe figure. Clear cold spots appear close to the centre 204 of the loadand adjacent to the middle of the edge 205 to the left in the figure.

Since the cavity of the microwave oven is fed sequentially in the twomodes which correspond to the heating patterns which are shown in FIGS.19 and 20, substantially more uniform heating of the load is thusobtained. Such sequential feeding may, for instance, take place on theanalogy of the method that has been described in connection with FIGS.12-14.

After having read and understood the description of the presentinvention, those skilled in the art will, by means of calculations andexperiments, find a wide range of possibilities of combining thelocation of the feeding ports and different heating patterns of thecavity modes, the combinations resulting in a microwave oven accordingto the present invention having superior performance and functions on acomparison with microwave ovens according to prior-art technique.

Although the invention has been described with reference to specificembodiments, it is understood that various modifications and additionsmay be made within its scope. The scope of the invention is defined in,and should be limited only by, the appended claims.

1. A microwave oven comprising a cavity that is defined by an enclosingsurface and adapted to receive a load to be heated, and a microwavesource which is connected to the cavity for feeding microwaves to thecavity, which cavity is provided with at least two feeding ports whereinthe cavity is designed to support at least two modes which areseparately feedable through at least a respective feeding port and theseparately feedable modes exhibits essentially non-overlapping heatingpatterns, and each one of the feeding ports is arranged to feed energyto a respective predetermined mode in the cavity, while feeding ofenergy to a mode other than the predetermined associated mode isessentially prevented, the feeding ports being placed at such a locationthat mainly one single mode is feedable by means of each feeding portand a transmission line in the form of a stripline or a microstrip,wherein the transmission line has a slot in its ground plane at alocation that at least partly overlaps the feeding port at the same timeas the transmission line exhibits a maximum current at the slot, wherebythe magnetic field in the transmission line is let into the cavity and,thus, feeds energy to it.
 2. A microwave oven as claimed in claim 1,wherein each feeding port is provided at a location where apredetermined mode in the cavity exhibits an essentially maximumamplitude of the magnetic field.
 3. A microwave oven as claimed in claim1, wherein the ground plane of the transmission line is short-circuitedwith its conducting plane by means of a loop which protrudes into thecavity through the feeding port, whereby an H-loop is formed which feedsenergy into the cavity.
 4. A microwave oven as claimed in claim 3,wherein each feeding port is placed at a location where a predeterminedmode in the cavity exhibits an essentially maximum amplitude of themagnetic field.
 5. A method for heating by means of energy in the formof microwaves, a load in the cavity of a microwave oven, wherein a firstfeeding port feeds energy to a first predetermined mode in the cavity,and a second feeding port feeds energy to a second predetermined mode inthe cavity in such a manner that the first mode is essentiallyunaffected by the energy which is fed from the second feeding port, andthe second mode is essentially unaffected by the energy which is fedfrom the first feeding port, wherein the first and the second feedingport sequentially feeds microwaves to the cavity in such a manner thatonly one of the first and second feeding port feeds energy into thecavity at any one instant, wherein the modes have essentiallynon-overlapping heating patterns, whereby a first mode provides heatingof mainly a first subset of the load and a second mode provides heatingof mainly a second subset of the load, the subsets being essentiallynon-overlapping, wherein reflected microwave power from at least one ofthe feeding ports is measured, and wherein the result of the measurementis used for controlling the feeding of microwaves to the cavity.
 6. Amethod as claimed in claim 5, wherein the feeding to the cavity iscontrolled in such a manner that the reflected power from the cavity isminimised.
 7. A method as claimed in claim 5, wherein a temperatureprofile of the load is measured, and wherein the result of themeasurement is used for controlling the microwave feeding to the cavity.8. A method as claimed in claim 7, wherein the feeding to the cavity iscontrolled in such a manner that an even temperature profile of the loadis promoted.
 9. A method as claimed in claim 5, wherein feeding ofenergy to the cavity is provided by means of a single microwave sourcehaving an essentially fixed emission frequency, the microwave sourcebeing connected to all the feeding ports.
 10. A method as claimed inclaim 9, wherein the microwave source is connected to the feeding portsby means of a network of transmission lines, preferably stripline ormicrostrip, the microwave power that is emitted by the microwave sourcebeing directed to the intended feeding ports by means of passivecomponents, such as directional couplers and/or circulators which arearranged in the network.
 11. A method as claimed in claim 9, wherein themicrowave source is connected to the feeding port by means of a networkof transmission lines, preferably stripline or microstrip, the microwavepower that is emitted by the microwave source being directed to theintended feeding ports by means of passive components, such asdirectional couplers and/or circulators which are provided in thenetwork.
 12. A method as claimed in claim 5, wherein feeding of energyto the cavity is provided by means of one single microwave source havinga variable emission frequency, the microwave source being connected toall the feeding pots.
 13. A method as claimed in claim 12, wherein thereflected power from the cavity is reduced to a minimum by the emissionfrequency of the microwave source being tuned to a frequency which givesa high coupling factor to the cavity.
 14. A method as claimed in claim12, wherein at least a first and a second feeding port are drivensequentially by the microwave source, and the microwave source, whendriving the first feeding port, is tuned to a first emission frequencyand, when driving the second feeding port, is tuned to a second emissionfrequency.
 15. A method as claimed in claim 5, wherein the feeding ofenergy to the cavity is provided by means of a plurality of microwavesources, each one having an essentially fixed emission frequency, andwherein each microwave source is connected to one or more feeding portscorresponding to a predetermined mode and frequency in the cavity asregards a respective source.
 16. A method as claimed in claim 5, whereinfeeding of energy to the cavity is provided by means of a plurality ofmicrowave sources having a variable emission frequency, and wherein eachmicrowave source is connected to one or more feeding ports correspondingto a predetermined mode and frequency in the cavity as regards arespective source.
 17. A method as claimed in claim 16, wherein theemission frequency of the respective microwave sources is controlled insuch a manner that the reflected power from the cavity of the microwaveoven is reduced to a minimum.
 18. A method as claimed in claim 16,wherein the power which is emitted from a respective microwave source iscontrolled in such a manner that uniform heating of a load placed in thecavity of the microwave is promoted.
 19. A method as claimed in claim18, wherein the control takes place on the basis of the result of ameasurement of the temperature distribution in the load.
 20. A method asclaimed in claim 19, wherein the measurement is carried out by means ifIR sensors.